Marine fouling is a major problem in the transport of materials worldwide as it raises fuel consumption by as much as 30%. Environmentally friendly coatings that protect the hulls of ships below the waterline against fouling by seaweed, barnacles, and other organisms are currently sought by the shipping industry. Fouling by these organisms produces additional drag on the ship, increasing the operating and maintenance costs. Currently, antifouling paints containing tin and copper biocides are used because of their effectiveness against most forms of fouling. However, these biocidal organometallic compounds that are environmentally persistent cause damage to the ecosystem, and enter the food chain. The ban on tributyltin (TBT) antifoulants by the International Maritime Organization will be effective in 2008, and copper-based coatings are expected to face similar restrictions in the near future.
Non-toxic “fouling-release” or “fouling-repellant” coatings are desirable alternatives to coatings that are biocidal. Silicone-based paints that belong to the former category are commercially available, but do satisfy many of the desired performance characteristics. The soft silicones do not withstand the rigorous demands of the marine environment, do not sufficiently and consistently self-clean, or, due to polymer restructuring or other degradation pathways, lose many of the desirable surface properties with time and exposure to marine organisms.
Current understanding of antifouling materials is that the most effective copper-free fouling control systems are low surface energy coatings, namely silicone or fluoropolymer based coatings that minimize the adhesion strength between fouling organisms and surface. For extended performance life, these coating systems must have controlled and stable surface energy and composition, have elastomeric properties, and adhere well to the substrate. To date, several fouling release (FR) coating systems are commercially available, mostly based on silicone polymers, yet none meet all of the desired performance characteristics. Many commercially available coating systems lack the toughness required to withstand the rigorous physical demands of the marine environment, do not sufficiently and consistently self-clean, and due to polymer restructuring or other degradation pathways, lose many of the desirable surface properties with time and exposure to the marine environment.
It is now apparent that adhesion strength of hard fouling organisms is proportional to (γE)1/2, where γ is the surface energy and E is the modulus of the surface. For this reason, silicone elastomers are, as of now, the only commercial environmentally benign fouling release coatings, because they possess both low modulus and low surface energy. The efficacy of silicone polymers is lower than that of biocide-containing antifouling paints, and regular mechanical cleaning (scrubbing) of the coated surface is required, adding to the operating expense. It would be advantageous to use fluorinated materials to lower the surface energy of a coating and decrease the adhesion strength, but these materials are typically hard and brittle with high moduli and do not provide good control of biofouling. Thermoplastic elastomers such as styrene-ethylene/butylene-styrene (SEBS) block copolymers offer good mechanical properties (low E), but not the desired surface energy.
Poly(ethylene glycol) (PEG) is commonly known by the biomaterials community to have good fouling resistance. The polymer has many useful properties such as low protein adsorption, good stability, low toxicity, and is, in general, compatible with biological processes as well as the human body. For fouling-release applications, such materials are lacking in several critical properties such as mechanical behavior, long-term stability, and ease of application, which limits their use as coating materials. As with biomedical surfaces used in the body, the first event in biofouling in the marine environment is adsorption of a conditioning film that includes proteins and glycoproteins.
What is needed is a material that lowers the strength of adhesion between the fouling organisms and the surface such that an environmentally friendly coating is provided that not only decreases the accumulation of the organisms, but also aids their removal. The diversity of fouling organisms and environmental conditions worldwide makes the task of developing a coating that resists fouling and/or self-cleans challenging, and novel non-toxic solutions are urgently needed.